This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Ming explored the use of several different K emission X-ray sources (W, Mo, Rh) to improve the contrast-to-noise for small animal digital subtraction angiography. Use of a K emitter will improve contrast to noise by: 1) flux production of the K emitters is much more efficient at low energies than Bremstrhalung;2) the quantum detection efficiency of the detector phosphors is much higher at lower energies (detecting phosphors must be thin to limit resolution degradation from light scatter);and 3) photoelectric absorption differences between soft tissue, calcification and iodine/gadolinium (potential vascular contrast agents) increase at lower energy. Based on simulation results, Ming enhanced our current micro-x-ray system for small animal digital subtraction angiography.